1. Field of the Invention
The present invention relates to an imaging device and a focusing control method.
2. Description of the Related Art
In recent years, according to the increase in resolution of an imaging element such as a charge coupled device (CCD) image sensor or a complementary metal oxide semiconductor (CMOS) image sensor, demand for information devices having an imaging function, such as a digital still camera, a digital video camera, a mobile phone such as a smart phone, or a personal digital assistant (PDA) has rapidly increased. Such an information device having an imaging function is referred to as an imaging device.
In such an imaging device, as a focusing control method for focusing on a main subject, a contrast auto-focus (AF) method or a phase difference AF method (for example, see JP2010-91991A, JP2012-8212A, and JP2001-330882A) is employed.
As a solid-state imaging element mounted in an imaging device that performs focusing control by a phase difference AF method, for example, a solid-state imaging element in which pairs of phase difference detection pixels in which light-shielding film openings are eccentric in reverse directions are discretely provided over an entire imaging surface is used.
A phase difference detection pixel has a sensitivity lower than that of a normal imaging pixel in which light-shielding openings are not eccentric with respect to a photoelectric conversion unit. Thus, in a case where a subject is placed in a low-intensity light, a signal level obtained from a phase difference detection pixel is reduced. In order to compensate for this reduction of the signal level, if a correlation operation is simply performed through a gain-up process, an error occurs in the correlation operation result.
JP2010-91991A discloses a configuration in which output signals in adjacent phase difference detection pixels are added up and a correlation operation for determining a defocus amount is performed using signals after the addition.
Further, JP2012-8212A discloses a configuration in which signals of phase difference detection pixels disposed at the same position in plural captured image signals obtained through consecutive imaging at a low luminance or with a large amount of defocus are added up and a defocus amount is calculated through a correlation operation using signals after the addition.
According to the configurations disclosed in JP2010-91991A and JP2012-8212A, it is possible to increase the intensity of signals by adding up signals of the phase difference detection pixels, and thus, it is possible to perform a focusing control with high accuracy even with respect to a dark subject.